According to the present invention, a circuit board and a method for assembling electronic devices, in particular an effective and simple solution for assembling electronic devices onto a circuit board having dense soldering pads and/or dense traces is disclosed.
Due to the facts that surface-mounting electronic devices onto the surface of a circuit board eliminates drilling of the circuit board as well as considerably increases the circuit density, and that soldering paste can be precisely applied over soldering pads by screen printing, Surface Mounting Technology (SMT) has become the primary method for assembling electronic devices onto circuit boards.
However, the trend of electronic industry is to produce electronic devices, such as integrated circuits ("IC's"), having considerably more and denser contacts, which require more, smaller, and denser soldering pads on the circuit board. The gaps between neighboring soldering pads and/or distance between soldering pads and neighboring traces must be reduced as a result of such smaller soldering pads, on which only tiny amount of soldering paste is applicable so as to eliminate the possibility of joining neighboring soldering pads or neighboring traces that ultimately result in short-circuits. Nevertheless, the tiny amount of soldering paste being applied onto the smaller soldering pads is insufficient to facilitate ideal soldering joints and thus reduces the reliability of soldering, and further deteriorates the quality of the entire circuits after completion.
To ensure the reliability of soldering, the measure generally adopted by the industry relates to limiting soldering pads to a predetermined dimension. However, such a limitation prevents further development of denser soldering pads and/or traces printed on the circuit board, and thus indirectly impedes development of electronic industry. An alternative measure adopted by the industry includes applying solder directly over the soldering pads, placing contacts of devices on the soldering pads plated with solidified solder, and melting the solder to connect the contacts and soldering pads. However, such a measure frequently causes bending of the contacts due to inconsistent thickness in solder plating, while controlling thickness of solder plating involves great difficulty.
An effective soldering solution for high-density soldering pads and/or closely neighboring soldering pads and traces is thus immediately required.